The permanent magnet synchronous motor (PMSM) is becoming widely accepted as one of the most viable choices in electric motor motion control. Due to the advent of rare earth and ceramic magnets, large torque to inertia ratios may be achieved with this design. In addition, coupled with this attractive feature is the absence of a need for mechanical commutation by slip rings, brushes and the like, due to the development of the resolver to digital converter and other power semi-conductor technologies. These developments allow the motor to be optimal in the sense that the best features of the separately excited direct current motor and the synchronous motor are present in this relatively new design. These motors are sometimes collectively referred to as brushless direct current motors with sinusoidal back EMF or surface permanent magnet synchronous motors.
Whited U.S. Pat. No. 4,447,771 describes a control system for synchronous brushless motors in which the motor windings are energized by sinusoidal excitation currents synthesized from pre-recorded sine values. An excitation current is generated in response to the position of the rotor relative to the stator windings in an attempt to create a rotating magnetic field synchronous with the rotation of the rotor. Control is achieved by comparing actual speed to desired speed and generating a signal.
In the above described control system, however, torque angle is controllable merely as a function of two (2) parameters, i.e., the rotor position with respect to the stator and the desired speed. Torque angle does not vary as a function of load and this causes the motor to operate at less than its most efficient operating point. Brown, et. al., U.S. Pat. No. 4,490,661 provides control of the angle between the rotating field created by permanent magnets and the rotating field created by the stator windings. The relationships between these two magnetic fields is controlled as a function of load and speed to provide what is suggested to be an optimum operating point at all times. A solution to this problem involves the use of sinusoidal excitations. Specifically, the sine waves are synthesized incrementally according to rotor position and thus the frequency of the excitation current is synchronous with the motor rotation. The phase of the excitation is a function of rotor position with respect to the stator. The method of control is a complicated method using a memory bank of torque angle factors with values created from pre-recorded digital sine values. Brown, et. al. does not provide for a simple solution to a difficult problem.
Peterson, et. al., U.S. Pat. No. 4,546,293 discloses the use of phase advance pulse width modulated current signals. A phase advanced circuit is coupled to receive a pulse wave-form signal for the motor rotor speed detecting unit and phase advance the pulse wave-form signal as a pre-determined function of motor speed to thereby cause the fundamental current wave form to be advanced. This compensates for fundamental current waveform lag due to motor winding reactance which allows the motor to operate at higher speeds than the motor is rated while providing optimal torque and therefore increased efficiency.
Glennon, et. al., U.S. Pat. No. 4,608,527 discloses a motor winding transformer circuit coupled to motor windings to provide an output signal representative of the magnitude and direction of the real current present in the windings. At the same time, a rotor speed position sensing circuit provides an output signal representative of the motor speed and direction. A load is coupled to the motor to be positioned by the motor in response to a load position command signal such that the load control is efficiently accomplished by the detection of a real current error signal which causes a phase advanced motor voltage waveform from the motor power supplied to achieve the desired load control.
Meshkat-Razavi U.S. Pat. No. 4,651,068 advances the phase of the control signal in a nonlinear manner, based in part at least on the phase lag due to inductance of the motor at high commutation frequencies corresponding to high motor velocities. This patent attempts to compensate for the command signal shape and phase to provide a relatively sinusoidal drive current and torque distribution over the entire operating frequency and torque loads. This is accomplished by amplitude compensation to control current in the windings. A microprocessor is disclosed for accomplishing the control described in this patent.
Lovernich U.S. Pat. No. 4,761,598 measures and adjusts the torque angle between the rotating magnetic field and the actual position of the rotor. This invention contemplates the maintenance of standard current at substantially constant values so that rotor torque and motion are controlled by monitoring and controlling the torque angle.
Dishner, et al. U.S. Pat. No. 4,835,448 employs a control system in which a run circuit compares a periodic signal representing motor speed at a maximum desired phase advance to a phase advance command signal to develop switching signals.
Aiello U.S. Pat. No. 4,884,016 addresses the problems of "six step" control to provide current command signals to a power amplifier that are converted into analog signals from the output of a read-only memory device that has been programmed with multiple sets of sinusoidal data.
Devitt, et al., U.S. Pat. No. 4,942,344 also relates to torque angle shifting for brushless DC motors using electronic commutations. Devitt, et al. provides a control method for eliminating the amount of torque angle shift to the minimum necessary in brushless motors. Means are provided for continuously comparing instantaneous parameter values with values corresponding to full utilization of a motor's supply voltage, and means responsive to that comparison for continuously transmitting a signal to a torque angle shift implementation means to thereby shift a torque angle closer to full utilization of the voltage supply.
Other related patents are Rees U.S. Pat. No. 5,006,774; Bardelang, et. al., U.S. Pat. No. 5,059,878; Stacey U.S. Pat. No. 5,113,125; and Toshihiko U.S. Pat. No. 5,229,693. All of these Patents provide additional control circuits or suggested methods for improving permanent magnet synchronous machines.
The difficulty with all of the aforementioned references is that the proposed control schemes are complicated and require transformation, on-line between rotating and stationary frames and the like and in some cases require replacement of existing linear controllers.
Accordingly, it is an object of this invention to provide a control circuit for use with electric motors which provide a substantial decrease in required current per unit of torque to thereby decrease loses via ohmic heating, and thereby extend the operating envelope of the machine.
Other objects will appear hereinafter.